Process of treating magnesium-bearing aluminum base alloys with fluoroborate



United States Patent Ofiice Patented May 5, 1959 PROCESS OF TREATINGMAGNESIUM-BEARING lrZLXUMINUM BASE ALLOYS WITH FLUOROBO- Spencer R.Milliken, Lower Burrell Township, Westmoreland County, Pa., assignor toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Application March 26, 1958 Serial No. 723,990

a 8 Claims. (Cl. 1486.27)

This invention relates to the thermal treatment of articles of aluminumbase alloys containing magnesium. More particularly, it is directed to atreatment for aluminum-magnesium alloys to prevent subsequentatmospheric and high-temperature oxidation and corrosion, and tofacilitate degassing of finished and semi-finished articles of suchalloys.

This application is a continuation-in-part of my co- 18, 1957, nowabandoned.

Thermal treatments, such as preheating for hot-working, annealing,solution heat-treatment and aging, are generally employed during thefabrication of products from aluminum and aluminumbase alloys. Thesetreatments are usually carried out in air atmosphere furnaces. In an airatmosphere which has not been dried and at temperature above 800 F.butbelow the melting point of the alloy, aluminum-magnesium alloyarticles tend to blister and stain, resulting in an objectionableappearance and sometimes a considerable decrease in mechanical strength.This effect is referred, to as high-temperature oxidation.

Some finished and semi-finished articles of aluminummagnesium alloyshave been found to contain appreciable quantities of hydrogen, which maygive rise to objectionable discontinuities in the metal structure. Thesediscontinuities are generally quite easily detectable by ultrasonictesting techniques. It has been proposed to heat such articles in airfor prolonged periods of time to diffuse hydrogen from the metal intothe surrounding atmosphere. However, it has been found that the presenceof small quantities of moisture in the surrounding atmosphere results inthe failure of such degassing. This failure to degas may be explained byreaction between the metal surface and moisture forming metal oxides anda high hydrogen partial pressure at the metal surface which preventsloss of hydrogen from the metal into the surrounding atmosphere. In somecases, the hydrogen partial pressure may be sufficiently high to causeadditional gassing of the metal.

To minimize high temperature oxidation, various methods have beenemployed to protect these alloys at temperatures above 800 E. US. Patent2,092,033 to Stroup discloses the additionof fluorine-containingsubstances 1 in vapor form to a non-dried air atmosphere to provide aprotective environment for such alloys. The vapor is convenientlygenerated from solid compounds which decompose at the elevatedtemperatures, above about 800 F. The metal is usually charged to afurnace already heated to the desired high temperature and containingthe protective atmosphere or supplied with the vaporgenerating fluoridecompounds if the protective atmosphere has not been previously provided,and it is quickly l heated to the desired temperature.

US. Patent 2,379,467 to Abbe discloses a protective treatment foraluminum alloy forgings in which an aqueous solution of sodiumfluoroborate is applied to the forging which is then dried prior tobeing heated to the pending application, Serial No. 684,642, filedSeptember solution heat treatingtemperature. The coated forging isquickly heated to the required temperature in accordance withconventional practice.

Although these processes have been helpful in many cases, they have noteliminated altogether the undesirable blistering and staining whichoften occurs on articles of aluminum-magnesium alloys, nor have theyproven effective in reducing the gas content of the metal.

It has been found that the blistering and staining of such alloys isapparently related to the oxidation of magnesium atoms at the externalsurface of the article and that the particles of these oxides or otheroxy-compounds act as focal points or channels for further oxidation onand within the surface. This oxidation is apparently caused by theattack of water upon the surface, oxidizing the aluminum and magnesiumand releasing hy drogen.

It has further been determined that above a certain criticaltemperature,there is a rapid increase in the rate of magnesium atom diffusion to thesurface and concomitant oxidation of the magnesium. Whereas in previousmethods oxidation has been inhibited by providing a protectiveatmosphere above that critical temperature, it has now been discoveredthat, for complete protection of aluminum-magnesium alloys, they must besubjected to preliminary vapor treatment below that temperature, moreparticularly, at a point :below the onset of any substantial amount ofmagnesium oxidation.

It is an object of this invention to provide a preliminary thermaltreatment for aluminum-magnesium alloy articles which substantiallyprevents high-temperature oxidation.

Another object is to provide a treatment for aluminummagnesium alloyarticles which will promote subsequent degassing of such articles in anatmosphere containing moisture.

It is also an object to provide a treatment for such articles whichdevelops a surface condition that permits storage in industrialatmospheres for prolonged periods of time without corrosion.

A further object is to provide surface protection on aluminum-magnesiumalloy articles which is stable at elevated temperatures in the presenceof oxygen and moisture.

It has been found that the foregoing objects can be attained by a methodof treating finished or semifinished articles of aluminum "base alloyscontaining magnesium wherein the articles are coated with an organicammonium fiuoroborate which yields boron trifluoride when heated to atemperature between 400 F. and 750 F., and heating the coated article toa temperature between 400 F. and 750 F., but in all cases below thecritical oxidation temperature of the alloy, for a period of timesufficiently long to cause substantial disappearance of the organicsubstance whereby a protective film is developed on the surface of thearticle which is substantially free from organic material. The alloyarticles which have been treated in this manner strongly resistoxidation at temperatures above 800 F. and up to the melting point ofthe lowest melting point constituent in the alloy which is oftenreferred to as the temperature of incipient fusion. In addition thetreated articles may be degassed by subsequent heating for prolongedperiods of time, even in the presence of moisture.

The critical oxidation temperature of an aluminummagnesium alloy isdefined as the lowest temperature at which the formation of magnesiumoxide can be detected by electron diffraction techniques. This may be aparticular temperature or a very narrow temperature range of the orderof 20 F., and hence the term is intended to cover both conditions. It isdirectly dependent upon the humidity of the furnace atmosphere, and thealloy composition, primarily the magnesium content. Although it can bedetermined readily for each set of conditions, it has been observed thatthese points fall within the range of 575 to 750 F., and generally 600to 675 F.

- Generally, the electron diffraction determination involves heatingsamples in air to various temperatures and under various conditions ofhumidity and time. After such treatment, the samples are exposed to anelectron beam in diffraction apparatus wherein the beam is refiectedfrom the surface of the samples and a pattern ismade. upon a film as inX-ray diffraction studies. By comparing the pattern with that of a knownsample, the nature of the substances on the surface of the metal samplecan be determined. This diffraction pattern provides. a qualitativeindication of the presence of magnesium oxide.

The term. aluminum-magnesium alloy as herein employed refers to analuminum base alloy containing on a weight basis 0.1 to percentmagnesium, with or without the presence of other elements, such as from0.1 to 12 percent copper or 0.25 to 14 percent silicon or 0.1 to.percent zinc or 0.1 to 3 percent manganese, or combinations of two ormore of these elements. Any of the foregoing alloys may also contain oneor more of the following elements, often referred to as hardeners, inthe following percentages:

0.05 to 0.5 percent chromium 0.01 to 0.5 percent titanium 0.25 to 2.5percent nickel 0.01 to 0.5 percent boron 0.002 to 2 percent beryllium0.1 to 0.5 percent molybdenum 0.1 to 0.5 percent zirconium 0.1 to 0.5percent tantalum 0.1 to 0.5 percent colombium 0.1 to 0.5 percent cobaltHowever, the total amount of the latter elements should not exceed about3 percent.

The term aluminum base alloy as used herein refers to those compositionswhich contain at least 50 percent by weight of aluminum.

The term organic boron trifiuoride-carrier compound as used hereinrefers to organic ammonium fluoroborates which yield boron trifiuoridein a reactive state between 400 and 750 F. These compounds'may bedescribed as the products of reaction between fluoroboric acid, HBF andthe constituent amine or amide. Generally, it is preferred to usecompounds which decompose or yield boron trifiuoride above 400 F. butbelow 575 F., the lowest critical oxidation temperature observed for analuminum-magnesium alloy. The boron trifiuoride, or substance containingboron trifiuoride in a reactive state, is generally produced by thepyrolytic decomposition of. the carrier compound. Compounds of thisgroup may beused singly or in combination.

Examples of such carrier compoundsare the. aliphatic ammoniumfluoroborates, di-n-butyl ammonium tetrafiuoroborate, n-octadecylN,N,N,trimethyl tetrafiuoro borate, and di-n-amyl ammonium tetrafiuoroborate.

It has been found that the boron trifiuoride-carrier compounds shouldyield substantially no boron trifiuoride below about 400 F. since theprotective film, if any, developed below this temperature isconsiderably less effective as an inhibitor than that produced by borontrifiuoride above 400 F., possibly due to the formation of a differentcrystalline structure in the surface reaction product. Nevertheless, thecarrier compound may decompose over a wide temperature range above 400F. as is generally experienced in commercial products of this type whichcontain. impurities. However, some decomposition may commence below 400F., but the amount is small and can be disregarded.

The organic boron trifiuoride-carrier compound is preferably applied tothe surface of the aluminum-magnesium alloy in a solution, although itmay be applied in a molten or liquid state. The solution may be appliedby spraying, swabbing, or immersing the article. The preferredembodiment of this invention utilizes an alcoholic or aqueous solutionof the organic boron trifiuoridecarrier compound containing at least 0.4percent by Weight of boron trifiuoride and preferably about 2.6 percent.In addition, wetting or other surface active agents may be added toimprove the application.

The coated articles are heated to a temperature of 400 F. or above wherethey are maintained until the organic substance substantially disappearsand the fluoride film, which is substantially free from organicmaterial, is formed. Periods from one to forty-five minutes have beensatisfactory to accomplish this purpose. Treatment periods of less thanone minute do not permit development of a satisfactory film orsubstantial disappearance of the organic coating while treatments inexcess of forty-five minutes do not provide any additional protection'or'offer any further benefit. Generally, periods from one to twentyminutes are sufiicient; however, longer times of treatment are desirablein the case of alloys particularly susceptible to oxidation or when thesurface oxide film is of greater thickness than that normally developedby exposure to the atmosphere.

The alloy articles treated in accordance with this in vention may besubjected immediately to higher heattreating temperatures or may beremoved for storage or subsequent heat treatment. The protective filmdeveloped effectively prevents atmospheric corrosion by industrial fume.

The present process may be practiced in an untreated air atmosphere,i.e., a normal air atmosphere as commonly employed in heat-treatingfurnaces. No drying of the air need be undertaken as moisture can betolerated; in fact, the invention has been successfully employed even infurnace atmospheres having dew points on the order of F. Also,observations indicate that more satisfactory results are obtained whenthe furnace atmosphere contains at least grains of water per cubic footof furnace volume, but it should not exceed about 45 grains per cubicfoot. In addition, the atmosphere may be contaminated with suchsubstances as sulfur dioxide, hydrogen chloride, ammonia, carbonmonoxide and carbon dioxide Without adverse effect upon the action ofthe boron trifiuoride. I

Gases which are inert toward the aluminum-magnesiumalloys may beemployed in place of air such as nitrogen, argon, helium andfuel gas.The term atmosphere as used herein includes air, the inert gases, orcombinations thereof.

Further, this invention is most effectively practiced when the alloyarticles have not previously been subjected to temperatures above theircritical oxidation temperature. Thepresence of magnesium oxide on thesurface, which has been produced. in preceding thermal treatments, isobserved to reduce the effectiveness of boron trifiuoride, presumablydue to the formation of. oxy-fiuoride complexes, but there is stillsubstantial benefit to be derived from the treatment of the presentinvention.

The temperature employed for the subsequent degassing treatment shouldbe above 750 F. but below the temperature of incipient fusion of thealloy. As is well known, the higher the temperature, the greater therate of diffusion and the shorter the time required. Generally, foraluminum-magnesium alloys a temperature of about 900 to 975 F. has beenfound satisfactory.

The period for the degassing treatment will depend primarily on thethickness of the article being treated (the shortest diffusing path).Generally at 940 F. periods in excess of several hours and up to 20 daysor more are necessary; for articles having a maximum crosssection of /2inch, periods in excess of 16 hours have been found adequate. Athickness of 1 inch will generally require a period in excess of'40hours at the same temperature; to days may be required forcrosssections, i.e. thicknesses, larger than 3 to 4 inches. Generally,the time required for the treatment will be related to the rate ofhydrogen difiusion in the alloy at a given temperature.

Subsequent to the degassing treatment, the treated article should besubjected to a working step to effect some reduction in size, thushealing the voids left by the diffused hydrogen. The percentage ofreduction necessary will be determined by the nature of the article andits original gaseous content.

The term degassing, as used herein, contemplates both the removal of gasfrom the metal and the prevention of regassing of the metal.

The etficacy of the present invention is illustrated by the followingexamples:

Example 1 A lot of 0.01 inch foil test strips of an alloy consisting ofaluminum, 1.5 percent magnesium, 4.5 percent copper and 0.65 percentmanganese was divided into two groups of specimens and heat treated for16 hours at 920 F. in an air atmosphere having a dew point of 125 F. Thefirst group of specimens had been pretreated by swabbing with a solutioncontaining 7.5 percent by weight of din-butyl ammonium tetrafluoroboratein isopropyl alcohol and heated to a temperature of 500 to 550 F. for aperiod of five minutes. Upon removal from the heat-treating operation,the first group of specimens was found to be free from blistering andstaining but the untreated group was moderately blistered and had agrey-black stain.

Example 2 A lot of 0.01 inch foil test strips of. an alloy consisting ofaluminum, 2.5 percent magnesium and 0.25 percent chromium was dividedinto two groups of specimens and heated 16 hours at a temperature of 950F. in an air atmosphere having a dew point of 125 F. The first group ofspecimens had been pretreated by swabbing with a solution containing 10percent by weight of n-octadecyl- N,N,N, trimethyl ammoniumtetrafluoroborate in isopropyl alcohol and heated to a temperature of550 F. for a period of ten minutes. Upon examination the pretreatedgroup of specimens was found to be free from blistering and stainingwhile the untreated was moderately blistered and had a dark grey stain.

Example 3 A lot of 0.01 inch foil test strips of an alloy consisting ofaluminum, 1.0 percent magnesium and 0.3 percent manganese was dividedinto two groups of specimens and heated for 16 hours at a temperature of950 F. in an air atmosphere having a dew point of 125 F. The first groupof specimens had been pretreated by swabbing with a solution containing7 percent by weight of di-namyl ammonium tetrafluoroborate in isopropylalcohol and heated to a temperature of 575 F. for a few minutes. Uponinspection the pretreated group was free from blistering and stainingbut the untreated was moderately blistered and had a grey-black stain.

Example 4 A lot of sheet ingots of an alloy consisting of aluminum, 2.5percent magnesium, and 0.25 percent chromium was divided into two groupsof specimens and preheated for 30 hours at a temperature of 950 F. priorto hot-rolling into sheet. The first group had been pretreated byswabbing with a solution of 7.5 percent by weight of di-n-butyl ammoniumtetrafluoroborate in isopropyl alcohol and heated through a temperaturerange of 450 to 575 F. for a period of thirty minutes. Upon inspectionafter preheating, the pretreated ingots were found to be free fromstaining although the untreated had a greyblack to blue-black stainindicating heavy'oxidation.

Example 5 A lot of /2 inch thick specimens was taken from a forgingingot of an alloy nominally consisting of aluminum, 4.4 percent copper,0.9 percent silicon, 0.8 percent manganese and 0.4 percent magnesium.The ingot had been rejected because of porosity. The specimens weredivided into two' groups and heated at 950 F. for 16 hours in an airatmosphere having a dew point of about F. One group of specimens waspretreated by swabbing with a solution containing 7% percent by weightof di-n-butylammonium tetrafluoroborate in isopropyl alcohol and heatedat a temperature of 450 to 550 F. for about 15 minutes. The other groupreceived no preliminary treatment. Upon removal from the furnace, bothgroups of specimens were subjected to gas evaluation by rapidly heatingthe specimens under reduced pressure to magnify voids. Upon inspection,the treated group of specimens wasfound to be free from such magnifiedvoids whereas the untreated showedmoderate to considerable quantities ofsuch magnified voids. This test indicates that the treatment of thepresent invention enabled the degassing of even rejected metal byheating for extended periods of time in an atmosphere containingmoisture.

Having thus described my invention, I claim:

1. The method of treating finished or semi-finished articles composed ofaluminum base alloy containing from 0.1 to 15 percent magnesiumcomprising the steps of coating said articles with an organic ammoniumfluoroborate which yields BF when heated to a temperature above 400 F.but below 750 F., and heating said coated article to a temperaturebetween 400 and 750 F., but in all cases below the critical oxidationtemperature of said alloy, for a period of time sufliciently long tocause substantial disappearance of the organic substance.

2. The method of treating finished or semi-finished articles composed ofan aluminum base alloy containing from 0.1 to 15 percent magnesiumcomprising the steps of coating said articles with an organic substanceselected from the group consisting of di-n-butyl ammoniumtetrafluoroborate, n-octadecyl-N,N,N, trimethyl ammoniumtetrafluoroborate, and di-n-amyl ammonium tetrafluoroborate, and heatingsaid coated article to a temperature between 400 and 750 F., but in allcases below the critical oxidation temperature of said alloy, for aperiod of time sufiiciently long to cause substantial disappearance ofthe organic substance.

3. The method of treating finished or semi-finished articles composed ofan aluminum base alloy containing from 0.1 to 15 percent magnesiumcomprising the steps of coating said articles with an organic ammoniumfluoroborate which yields BF when heated to a temperature above 400 F.but below 575 F., and heating said coated article to a temperaturebetween 400 and 575 F. for a period of time sufliciently long to causesubstantial disappearance of the organic substance.

4. The method of treating finished or semi-finished articles composed ofan aluminum base alloy containing from 0.1 to 15 percent magnesiumcomprising the steps of coating said articles with an organic substanceselected from the group consisting of di-n-butyl ammoniumtetrafluoroborate, n-octadecyl-N,N,N, trimethyl ammoniumtetrafluoroborate, and di-n-amyl ammonium tetrafluoroborate, and heatingsaid coated article to a temperature between 400 and 575 F. for a periodof time sufiiciently long to cause substantial disappearance of theorganic substance.

5. The method of treating finished or semi-finished articles composed ofan aluminum 'base alloy containing from 0.1 to 15 percent magnesiumcomprising the steps of coating said articles with an organic ammoniumfluoroborate which yields BF; when heated to a temperature above 400 F.but below 750 F., and heating said coated article in an air atmosphereto a tempera- 7 ture between 400 and 750?- F., but in all cases belowthe critical, oxidation temperature of said alloy, for a period of timesufficiently long to cause substantial disappearance of the organicsubstance.

6. The method in accordance with claim 5 wherein the air atmospherecontains at least but not more than 45 grams of water per cubic foot;

7. The method of treating finished or semi-finished articles composed ofan aluminum base alloy containing from 0.1 to 15 percent magnesium toimprove the resistance to corrosion at room temperature and resistanceto oxidation at elevated temperatures, said method comprising the stepsof coating said articles with an organic ammonium fluoroborates whichyields BF when heated to a temperature above 400 F. but below 750 F.,and heating said coated article to a temperature between 400 and 750 F;,but in all cases below the critical oxidation temperature. of saidalloy, for a period of time sufiiciently long to cause substantialdisappearance oil the organic; substance whereby a. protective film isdeveloped onthe. surface: of said article that is substantially free.fromv organic material.

8. The method of treating finished or semi-finished articles composed.of. an aluminum base alloy containing from 0.1 to 15 percent magnesiumprior to degassing by thermal treatment at temperatures above 750 F. butbelowthe temperature of incipient fusion comprising the steps of coatingsaid articles with an organic ammoniumfluoroborate which yields BF whenheated to a temperature above 400 F. bntbelow 750 F., and heating saidcoated article to a temperature between 400 and 750 F., but in all casesbelow the critical oxidation temperature of saidalloy, for a period oftime sufficient- 1y long to cause substantial disappearance of theorganic substance.

No references cited.

1. THE METHOD OF TREATING FINISHING OR SEMI-FINISHED ARTICLES COMPOSEDOF ALUMINUM BASE ALLOY CONTAINING FROM 0.1 TO 15 PERCENT MAGNESIUMCOMPRISING THE STEPS OF COATING SAID ARTICLES WITH AN ORGANIC AMMONIUMFLUOROBORATE WHICH YIELDS BF3 WHEN HEATED TO A TEMPERATURE ABOVE 400*F,BUT BELOW 750*F., AND HEATING SAID COATED ARTICLE TO A TEMPERATUREBETWEEN 400 AND 750*F., BUT IN ALL CASE BELOW THE CRITICAL OXIDATIONTEMPERATURE OF SAID ALLOY, FOR A PERIOD OF TIME SUFFICEINTLY LONG TOCAUSE SUBSTANTIAL DISAPPEARANCE OF THE ORGANIC SUBSTANCE.